In the search for earth formations containing hydrocarbons, a well bore is drilled and instruments are passed along the well bore to make measurements. From the measurements, the formation lithology, porosity, and other factors from which the presence of hydrocarbons can be deduced are measured. More often than not, several different types of measurements are necessary for formation analysis and it is necessary to obtain multiple types of measurements with separate tools. When logging tools are run separately, there is always a problem of correlating the depth in addition to the time consumed in removing and rerunning other tools. The loss of valuable rig time is an important commercial consideration. Where a single tool obtains multiple measurements, there is the problem of communication of power and signals between the surface and the tool over the electrical logging cable. For nuclear well logging, it is desirable to use a type of cable as disclosed in a copending application entitled "Well Logging System and Method Using An Armored Coaxial Cable and Compensation Circuit", Ser. No. 192,883 filed Oct. 27, 1971 and which is assigned to the assignee of the present invention. This is a triaxial (or armored coaxial) type of cable and it has desirable bandwidth capabilities for enhancing the measurement and preserving the shape of electrical pulses generated in the downhole equipment. As would be expected with this type of cable which has a limited number of conductors, certain types of measurements cannot be simultaneously made. In nuclear logging, for example, it is desirable to have a capability for obtaining a carbon/oxygen log and a thermal neutron decay time log on one pass through the well bore. While the equipment for obtaining these logs is similar, there are different neutron pulse frequencies, operating voltages, voltages and timing functions, so that obtaining both logs with one run of the tool requires downhole switching and control functions.
In the practice of the present invention, a logging tool is provided with a high energy neutron generator (14 MeV) and a gamma ray detector. Thus, zones of interest can be found for further measurements related to carbon/oxygen content. This type of measurement is typically run at a much lower logging speed and uses a different neutron pulse duration and frequency.
There are other factors which differentiate a thermal decay tool from carbon/oxygen tool. For example, a C/O tool may have a source detector spacing up to four inches more than a spacing required for a thermal decay tool. In regard to neutron pulse frequency and neutron generator pulse width, a thermal neutron decay time tool can operate at 1 KHz for neutron burst repetition with an ion source duration of 75 microseconds. A C/O tool on the other hand, has a neutron pulse repetition frequency of about 20 KHz and a 20 microsecond ion source pulse width. The measurements of gamma rays for thermal neutron decay time measurements occur at a time after the neutron burst. On the other hand, measurements for C/O ratio are made during the burst of neutron energy since inelastic scattering is measured. One of the advantages of a C/O tool is that it is substantially salinity independent, whereas a thermal neutron decay time tool is limited to areas of fairly high salinity.